Lithium secondary batteries and lithium ion secondary batteries (hereinafter, referred to as secondary batteries) have a feature of being of a small size and having a large capacity. Therefore, secondary batteries are broadly used as power sources for cell phones, laptop personal computers and the like.
LiCoO2 is mainly utilized as a positive electrode active material of the secondary battery. However, LiCoO2 does not have sufficient safety in the charged state, and besides, the Co raw material is expensive. Therefore, search for novel positive electrode active materials replacing LiCoO2 has been made actively.
As a material having the same lamellar crystal structure as LiCoO2, use of LiNiO2 is being studied. However, although LiNiO2 has a high capacity, LiNiO2 has a lower potential than LiCoO2, and besides has a problem in the aspect of safety. Further, LiNiO2 is expensive because using much amount of the Ni raw material.
As another positive electrode active material, use of LiMn2O4 having a spinel structure is being actively studied. The Mn raw material is relatively inexpensive, and therefore has a merit in price. However, LiMn2O4 has a problem of occurrence of the performance deterioration along with the cycle and the capacity decrease at high temperatures. It is believed that this is caused by the instability of trivalent Mn, and the Jahn-Teller strain is generated in the crystal when the average valence of Mn ions changes between trivalence and tetravalence, and decreases the stability of the crystal structure.
Therefore, in order to enhance the reliability of a secondary battery, studies have been made in which the structural stability is improved by substituting trivalent Mn with another element. For example, Patent Literature 1 discloses a positive electrode active material in which trivalent Mn contained in LiMn2O4 is substituted with another metal. Specifically, Patent Literature 1 describes a secondary battery including, as a positive electrode active material, a manganese composite oxide having a spinel structure and represented by the composition formula LiMxMn2-xO4 (M is one or more selected from Al, B, Cr, Co, Ni, Ti, Fe, Mg, Ba, Zn, Ge and Nb, and 0.01≦x≦1). Patent Literature 1 specifically discloses an example of using LiMn1.75Al0.25O4 as a positive electrode active material.
However, in the case where trivalent Mn is substituted with another element, there arises a problem of a decrease in the discharge capacity. LiMn2O4 causes a valence change of Mn represented by the following formula along with charge/discharge.Li+Mn3+Mn4+O2−4→Li++Mn4+2O2−4 
As shown in the above formula, LiMn2O4 contains trivalent Mn and tetravalent Mn, and the discharge is caused by change of trivalent Mn therein to tetravalence. Therefore, substitution of trivalent Mn with another element inevitably results in a decrease in the discharge capacity. That is, if the structural stability of a positive electrode active material is attempted to be enhanced to improve the reliability of a secondary battery, the decrease in the discharge capacity becomes remarkable. Therefore, the satisfaction of the both is difficult. Particularly a positive electrode active material exhibiting a high reliability in the discharge capacity of 110 mAh/g or larger is difficult to obtain.
A positive electrode active material in which trivalent Mn contained in LiMn2O4 is substituted with another element as described above constitutes a secondary battery having a so-called 4-V class electromotive force. On the other hand, as a technology in a different direction, studies are made in which a part of Mn of LiMn2O4 is substituted with Ni, Co, Fe, Cu, Cr or the like to raise the charge/discharge potential to thereby increase the energy density. These constitute batteries having a so-called 5-V class electromotive force. Hereinafter, description will be made by citing LiNi0.5Mn1.5O4 as an example.
LiNi0.5Mn1.5O4, ideally, causes a valence change represented by the following formula along with charge/discharge.Li+Ni2+0.5Mn4+1.5O2−4→Li++Ni4+0.5Mn4+1.5O2−4 
As shown in the above formula, LiNi0.5Mn1.5O4 causes discharge by a change of divalent Ni to tetravalence. In such a way, use of Ni as an element participating in charge/discharge allows the operation at a high potential of 4.5 V or higher vs. lithium metal. Similarly in the case of substitution with Cr, a valence change of Cr from trivalance to tetravalence allows intercalation/deintercalation of lithium ions at a high potential of 4.5 V or higher vs. lithium metal. Patent Literatures 2 to 4 disclose examples of such positive electrode active materials. Use of such a positive electrode active material allows acquisition of an electromotive force of 4.5 V or higher vs. lithium and a high charge/discharge capacity.